The present invention relates to a distortion compensation output control circuit and a distortion compensation output control method in which distortion compensation and output control are performed.
FIG. 1 is a block diagram showing the configuration of a conventional distortion compensation output control circuit disclosed in Japanese Patent Gazette No. 2576357. In FIG. 1, 1 indicates an adder for adding a value of input electric power and a value of a part of output electric power negatively fed backed together. 2 indicates a variable attenuator for attenuating the electric power output from the adder 1. 3 indicates an electric power amplifier for amplifying the electric power output from the variable attenuator 2 and outputting output electric power set to a prescribed value. 4 indicates a variable attenuator for attenuating the output electric power output from the electric power amplifier 3 and negatively feeding back the attenuated output electric power to the adder 1. 5 indicates a control circuit for controlling the variable attenuator 4 so as to obtain the output electric power of the prescribed value and controlling the variable attenuator 2 so as to always set a loop gain of a feedback loop to a constant value. As is described above, the conventional distortion compensation output control circuit has forward side elements composed of both the variable attenuator 2 and the electric power amplifier 3, feedback side elements composed of both the variable attenuator 4 and the adder 1 and the control circuit 5. Also, in the conventional distortion compensation output control circuit, input electric power is amplified, output electric power set to a prescribed value is output, a feedback loop is formed to perform negative feedback of a part of the output electric power, and distortion occurring in the amplification of the input electric power is compensated.
Next, an operation will be described below.
Normally, in cases where a level (or value) of output electric power is changed in the conventional distortion compensation output control circuit, a level of the output electric power negatively fed back to the adder 1 is changed in the variable attenuator 4 to change a level of the output electric power. In this case, in the Patent Gazette, the variable attenuator 2 is controlled by the control circuit 5 according to a degree of change performed in the variable attenuator 4 so as to always set a loop gain in the conventional distortion compensation output control circuit to a constant value. Therefore, a stable output control can be always performed.
FIG. 2 is a view showing a quantity of distortion with respect to the output electric power of the electric power amplifier 3. In general, a distortion characteristic of the output electric power of the electric power amplifier 3 is not always constant. As shown in FIG. 2, a quantity of distortion is increased or decreased according to the output electric power. However, in the conventional distortion compensation output control circuit, even though the output electric power is controlled, the variable attenuator 2 is controlled so as to always set the loop gain to a constant value. In this case, even though the quantity of distortion is small and the value of the output electric power is low, the loop gain is still set to a constant value. Therefore, the setting of the loop gain to a constant value causes various problems such as the increase of noise electric power and phase oscillation peculiar to the conventional distortion compensation output control circuit. For example, it is allowed that a quantity of distortion compensation for the output electric power of a level B shown in FIG. 2 is lower than that for the output electric power of a level A shown in FIG. 2, and no distortion compensation is required of the output electric power of a level C shown in FIG. 2.
Also, FIG. 3 is a view showing a quantity of distortion with respect to the output electric power of the electric power amplifier 3 which has characteristics of a higher order function. As shown in FIG. 3, In dependence on the configuration of the electric power amplifier 3, there is a case where the electric power amplifier 3 has characteristics of a higher order function in which a quantity of distortion in the whole forward side elements is not constant with respect to the output electric power. In cases where the output electric power of a level A shown in FIG. 3 is changed to the output electric power of a level B shown in FIG. 3, a range of distortion compensation cannot be satisfied, and it is expected that an absolute quantity of distortion for the output electric power of the level B is higher than that for the output electric power of the level A. As is described above, in cases where a quantity of distortion is changed according to the output electric power, it is difficult to obtain stable performance.
Because the conventional distortion compensation output control circuit has the configuration described above, in cases where a quantity of distortion in the electric power amplifier 3 has characteristics of a higher order function with respect to the output electric power and is changed according to the output electric power, a problem has arisen that it is difficult to obtain stable performance.
Also, when the output electric power is controlled, the variable attenuator 2 is controlled so as to always set the loop gain to a constant value. Therefore, another problem has arisen that the increase of noise electric power and phase oscillation are caused.
The present invention is provided to solve the above-described problems, and the object of the present invention is to provide a distortion compensation output control circuit and a distortion compensation output control method in which a quantity of distortion is controlled according to output electric power while controlling the output electric power and stable performance can be obtained.
Also, the object of the present invention is to provide a distortion compensation output control circuit and a distortion compensation output control method in which a feedback loop is set to an off condition in cases where a large quantity of output control is performed and a quantity of distortion is small due to output electric power of a low value, and probability of causing problems such as the increase of noise electric power and phase oscillation is prevented.
A distortion compensation output control circuit according to the present invention, in which input electric power is amplified, output electric power of a prescribed value is output, a feedback loop is formed to perform negative feedback of a part of the output electric power and distortion occurring in the amplification of electric power is compensated, comprises an adder for adding a value of the input electric power and a value of the part of the output electric power negatively fed back together, first gain changing means for increasing or decreasing a value of electric power output from the adder, an electric power amplifier for amplifying electric power output from the first gain changing means and outputting the output electric power of the prescribed value, second gain changing means for attenuating the output electric power output fron the electric power amplifier and negatively feeding back the output electric power to the adder, a second control circuit for controlling the second gain changing means so as to obtain the output electric power of the prescribed value, and a first control circuit for controlling the first gain changing means so as to obtain a loop gain of the feedback loop corresponding to the prescribed value of the output electric power.
Therefore, a quantity of the distortion can be controlled according to the output electric power, and stable performance can be obtained.
A distortion compensation output control circuit according to the present invention further comprises a switching circuit for setting the feedback loop to the adder to an on condition or an off condition. The feedback loop is set to the off condition by the switching circuit in a case where the prescribed value of the output electric power is low and a quantity of the distortion in the output electric power is small, and the first gain changing means is controlled so as to obtain the output electric power of the prescribed value in the first control circuit.
Therefore, probability of causing problems such as the increase of noise electric power and phase oscillation can be prevented.
A distortion compensation output control method according to the present invention, in which input electric power is amplified, output electric power of a prescribed value is output, a feedback loop is formed to perform negative feedback of a part of the output electric power and distortion occurring in the amplification of electric power according to both a forward side gain and a feedback side gain is compensated, comprises the steps of controlling the feedback side gain so as to obtain the output electric power of the prescribed value, and controlling the forward side gain so as to obtain a loop gain of the feedback loop corresponding to the prescribed value of the output electric power.
Therefore, a quantity of the distortion can be controlled according to the output electric power, and stable performance can be obtained.
A distortion compensation output control method according to the present invention further comprises the steps of setting the feedback loop to an off condition in a case where the prescribed value of the output electric power is low and a quantity of the distortion in the output electric power is small, and controlling the forward side gain so as to obtain the output electric power of the prescribed value.
Therefore, probability of causing problems such as the increase of noise electric power and phase oscillation can be prevented.